The present invention relates to a resin composition suited for use as a paint composition which gives a coating having improved appearance and high hardness and excellent in weatherability, chemical resistance, solvent resistance and water resistance.
In recent years, in the field of paint technology, particularly for automobile finish, there is an increasing demand for a paint which gives an improved appearance of the paint film, i.e. improved smoothness and gloss. In order to comply with such a demand, it has been investigated to smoothen the paint film by controlling the flow characteristics during drying and curing of the paints or by reducing shrinkage during the drying or curing step. However, only few trials have been made to improve the gloss of the paint film by increasing the refractive index of the resin for paint. For instance, when the gloss of the coating is indicated by the Rs value (Gloss at 30 degrees measured by the Dorigon goniophoto meter produced by Hunter Lab.), it is difficult to provide a coating having an Rs value of not less than 86 unless the coating contains styrene. For example, the refractive indices of the homo-polymers of methyl methacrylate, butyl methacrylate, hexyl methacrylate, ethyl acrylate and butyl acrylate are so low as 1.490, 1.483, 1.481, 1.469 and 1.466, respectively, these monomers being generally used as the monomers for the preparation of acrylic resins for paint use. On one hand, although the fluorine-contained resins are improved in weatherability and attract public attention, the practical use thereof is delayed since they have low refractive indices ranging within 1.3 to 1.4 to make it hard to obtain paint films of good appearance. On the other hand, although the homopolymer of styrene has a high refractive index of 1.60, it is poor in weatherability and thus the use thereof for automobile finish is limited since the coatings applied on the automobiles must be durable for a long time.
It has generally been known that the interrelation between the chemical structure of an organic compound and the refractive index is indicated by the Lorentz-Lorentz's formula, i.e. the following formula (II): ##EQU1## wherein .eta..sub.D is the refractive index,
[R] is the molecular refraction, PA0 [R]=4.pi./3.multidot.N.sub.A .multidot..alpha.(where N.sub.A is the Avogadro number, and .alpha. is the polarizability.) PA0 V is the molecular volume, PA0 V=M/.rho. (where M is the molecular weight, and .rho. is the density)
Accordingly, in order to increase the refractive index of a compound, adopted is an approach in which the molecular refraction [R] is increased by introducing a chemical structure having a higher polarizability, or an approach in which the molecular volume V is decreased by introducing an atom which makes the density of the molecule to a higher value. In view of the foregoing standpoint, some organic optical materials having high refractive indices have been investigated in the field of plastics lens, and as the fruits of such investigations there are developed resins each containing a large amount of halogen atoms, such as chlorine and bromine, or a large amount of aromatic rings as the organic optical materials having large molecular refractions [R], and also developed are resins each containing a heavy metal, such as lead, barium or lanthanum as the organic optical materials having small molecular volumes V.
However, if a large amount of halogen atoms is included in a resin to provide a higher refractive index, the weatherability of the resin is deteriorated due to the chemical activity of the halogen atoms; whereas if a large amount of aromatic rings is included, the melting point of the monomer is raised to adversely affect the workability or operation efficiency at the polymerization step. on the other hand, the resins containing heavy metals are apt to breakdown at the points at which the metals are linked to the organic compounds, leading to poor chemical resistance, and thus the resins as such are not suited for use as paint applications.
Poly(meth)acrylate copolymers, prepared from monomer mixtures containing polycyclic (meth)acrylate monomers, have been proposed for use as optical materials for optical fibers, optical disks, optically sensible cards, plastic lens and transparent conductive sheets, since they are improved in transparency, heat resistance, chemical resistance, solvent resistance and mechanical strengths. More specifically, Japanese Patent Laid-Open Publication No. 8355/1988 discloses a (meth)acrylic ester comprised of a polycyclic alkyl (meth)acrylate; Japanese Patent Laid-Open Publication No. 141009/1987 discloses a poly(meth)acrylate copolymer having a particular intrinsic viscosity and glass transition temperature, the copolymer being comprised of a polymer prepared from a polycyclic (meth)acrylate monomer; Japanese Patent Laid-Open Publication No. 209114/1987 discloses a poly(meth)acrylate copolymer having a particular intrinsic viscosity and glass transition temperature, the copolymer being prepared by copolymerizing a polycyclic (meth)acrylate monomer with a (meth)acrylate monomer; and Japanese Patent Laid-Open Publication No. 141012/1987 discloses a poly(meth)acrylate copolymer having a particular intrinsic viscosity and glass transition temperature and containing substantially no gelled cross-linked polymer, the copolymer being prepared from a polycyclic (meth)acrylate monomer, a (meth)acrylate monomer and a polyfunctional (meth)acrylate monomer having 2 to 4 (meth)acryloyloxy groups in one molecule.
However, all of the poly(meth)acrylate copolymers described above are developed to produce molded articles used as optical materials, such as plastic lens or the like, and thus it is difficult to use them directly as the resins for paints. Particularly, when a polyfunctional (meth)acrylate is used as one of the polymerizable ingredients as taught by Japanese Patent Laid-Open Publication No. 141012/1987, the viscosity of the resultant copolymer per se becomes higher to make it impossible to use the same as the resin for paint applications. Under these circumstances, there is an increasing demand for the development of a paint containing, as the coating-forming ingredient, a resin which has a high refractive index and is improved in weatherability, chemical resistance and water resistance to give a coating of high hardness.